1. Field of the Invention
The present invention relates generally to controllable hydraulic dampers for vehicular suspension. In particular, the present invention is concerned with a low level damping valve mounted in series with a solenoid valve in a semi-active suspension actuator.
2. Description of the Related Art
For purposes of this specification, an active suspension system includes an actuator capable of modifying the force between a vehicular body and a wheel assembly in response to a control in real time to produce a desired suspension behavior. A semi-active suspension system is similar but uses an actuator (a damper) which provides only damping or dissipative forces in real time wherein the suspension system generates its own force. Additionally, an on/off semi-active damper has only two operating modes with fixed force/velocity damping curves: a first mode having a low damping force ("soft") when the solenoid valve is open; and a second mode having a high damping force ("firm") when the solenoid valve is closed. A continuously variable semi-active damper has a continuous range of damping modes between minimum and maximum settings.
Semi-active dampers include electric solenoid valves capable of switching a damper between operating modes at frequencies preferably at least twice the resonant wheel vibration frequency to achieve real time control of wheel and body motions. Semi-active dampers are also known as "fast shocks" and as "real time dampers."
Passive hydraulic dampers (shock absorbers and struts) include a reciprocable piston and piston rod slidably mounted in a fluid-filled cylinder. The piston divides the interior of the cylinder into upper and lower chambers. A valve assembly in the piston provides a restricted fluid path between the chambers. A reservoir tube concentrically mounts and cooperates with the cylinder to form a fluid reservoir. The lower end of the cylinder is closed by a compression or base valve assembly. During a compression stroke, fluid travels from the lower chamber through the piston valve assembly to the upper chamber. An amount of fluid equal to the increasing rod volume in the upper chamber travels through the base valve assembly to the reservoir. During a rebound stroke, fluid travels from the upper chamber through the piston valve assembly to the lower chamber. Also, fluid travels from the reservoir through the base valve assembly to the lower chamber to compensate for the rod volume. The restriction of fluid through the piston and base valve assemblies provides the level of damping in a damper.
A semi-active damper uses a bypass channel between the upper chamber and the reservoir in a well-known manner. As described above, a semi-active damper incorporates either an on/off or continuously variable solenoid valve assembly to control fluid flow in the bypass channel. When the solenoid valve assembly is opened, fluid travels from the upper chamber to the reservoir through the bypass channel. This redirection of fluid changes the damping characteristic of the damper to a "soft" setting, since the bypass channel permits fluid flow with less restriction than the piston and base valve assemblies. When the solenoid valve assembly is closed, fluid is forced to flow through the piston and base valve assemblies in the usual manner, resulting in a "firm" setting.
Semi-active dampers can be tuned so that the vehicle experiences a boulevard-type ride with operating in the soft mode. It has been determined that a certain amount of low damping is desirable at such conditions to improve vehicle handling.
The art continues to seek improvements. It is desirable to provide low level damping in a semi-active damper to enhance ride control.